The invention relates generally to the field of electrical drives. More particularly, the invention relates to techniques for detecting ground faults in a high resistance ground network system.
In the field of power electronic devices, a wide range of circuitry is known and currently available for converting, producing and applying power to loads. Depending upon the application, motor drive systems may include circuitry that converts incoming power from one form to another as needed by the load. In a typical drive system, for example, a rectifier converts alternating current (AC) power (such as from a utility grid or generator) to direct current (DC) power. Inverter circuitry can then convert the DC signal into an AC signal of a particular frequency desired for driving a motor at a particular speed. The inverter circuitry typically includes several high power switches, such as insulated-gate bipolar transistors (IGBTs), controlled by drive circuitry. Motor drive systems also often include power conditioning circuitry, including capacitors and/or inductors, which remove undesirable ripple currents on the DC bus.
Electronic drives may be used for a wide range of industrial applications, and the system configurations for such different applications and different mechanical functions may also vary greatly. In certain electrical systems, high resistance ground (HRG) configurations may be advantageous for the operation of the drive. An HRG system involves inserting a resistance between a neutral node and the ground of the input AC source of the system such that the system can operate under single ground faulted condition with a small non-destructive ground fault current. During the operation of the drive, ground faults may sometimes occur. For example, ground faults in an HRG system may result in high voltage levels in bus lines associated with the electrical drive and high voltage stresses in electrical drive components, which may eventually result in failures of the electrical drive or other system components. However, HRG faults are typically difficult to detect once a drive is in operation, as the small amount of fundamental frequency ground current is difficult to measure in the presence of high frequency electronic noise created by the drive, especially in larger electrical drives or during low output speed conditions.
Most conventional techniques for detecting ground faults on HRG networks are based on simplified impedance models for the transmission line. However, these techniques lack sufficient accuracy when applied to systems containing power converters. Some techniques involve adjusting fault detection processes for both high frequency and low frequency operating conditions of a drive in an HRG system. However, such techniques may also lack sufficient accuracy, as false triggers may be generated under different system configurations.
It may be advantageous to provide techniques for detecting presence and location of ground faults on HRG networks in power conversion circuits during and/or prior to operation of such systems.